Walking through the Terrabay neighborhood Monday morning, three San Bruno Mountain Watch members looked for clues to where a dirt biker is entering the park.

South San Francisco residents Loretta Brooks, Chuck Heimstadt and Ken Oborn, all members of a San Bruno Mountain Watch conservancy committee, noticed the illegal tracks a week ago.

Brooks says dirt biking can destroy lupine, a primary food source for the endangered Mission Blue Butterfly, which is native to the mountain.

"We are trying to find a way to protect open space around here," Brooks said.

The group suspects that the dirt biker entered the park at the end of Highcrest Lane, off of Hillside Boulevard, where there is parking and easy access. A sign clearly marks that motorized bikes are prohibited in the park.

The sun peaked out from behind the clouds as water trickled down the verdant canyon and ravens circled overhead.

While hiking up the steep ascent, Brooks and Heimstadt pointed out a variety of invasive species, like fennel, radish and Italian thistle.

"The whole food chain is interconnected," Heimstadt said. "By bringing in the non-natives, it breaks up that food chain."

Native species like Blue-Eyed Grass, Sticky Monkey, and California Aster also sprout along the trail, but in fewer numbers.

The group wants to help the San Mateo County Parks Department weed out invasive species in the area but is waiting on a permit.

Heimstadt said there are multiple paths leading up the mountain, but all of them are steep.

"This was never a built trail," he said. "It needs to be switch backed."

The couple looks forward to a proposed east-west bike trail that would go from Oyster Point Marina to the ocean at Fort Funston. A new trail would be built from the base of San Bruno Mountain at Sister Cities Boulevard up to the Ridge Trail.

Hiking up the spine of the canyon past the blooming yellow petals of San Francisco Wallflower, the bike tracks became visible. The tracks zigzag on the mountainside between some rock outcroppings.

Two hikers made their way down, and Brooks asked if they've heard any bikes.

"It's against the law and it can ruin the habitat," she said. The hikers said they haven't seen anyone riding here but will contact authorities if they do.

"There's enough erosion on this mountain without [dirt biking]," Heimstadt says, " and then it's unsightly."

He says the county may be able to install a gate that would stop dirt bikers from entering.

Elias Frangos, park aide for the parks department, said he hadn't received any reports of dirt biking but they will look into it.

"We can definitely go check it out and keep an eye over there," Frangos said.